All About Brake Fluid!
Brake fluid makes a huge impact on the performance of your motorcycle brakes! Here's a few things to keep in mind for that.
The Overview/Takeaway
- Run fluid with a higher temp rating than you need, to avoid frequent flushing.
- Fluid absorbs moisture, so flush the fluid in your brakes at least once per year, and don't keep leftover bottles of fluid for more than a few months.
KEEP READING BELOW for more details on brake fluid!
MOISTURE CONTAMINATION, BOILING, & FLUID BREAKDOWN
Brake fluid is hydroscopic so it absorbs water/moisture. As the water content increases, the boiling point decreases and the fluid becomes less effective. This happens to fluid that is in your motorcycle's brake system, as well as the leftover fluid in the bottle that you didn't use. Humid climates will reduce that timeframe as well.
The biggest issue your brake fluid faces is heat, which causes it to break down and even boil if the temperature is high enough. If you are more aggressive with your brakes (like racing, track riding), you'll want to start with fluid rated to a higher temperature rating and plan to flush your fluid more frequently. Stunting and aggressive canyon riding can also benefit from using fluid with a little higher temp rating.
When brake fluid boils, it's creating air in the hydraulic system that is your brakes. This hydraulic system provides more precise movement of the components (and feeling in the lever) because the fluid is NOT compressible. Air IS compressible so when the fluid boils and air is created, it creates that spongey feeling in the lever. This is the same reason why poorly-bled brakes also have a spongey feel.
BOILING POINTS, WET & DRY
The "dry boiling point" is when the fluid is new and has not yet absorbed any moisture. The "wet boiling point" is after the fluid has absorbed 3.7% moisture. Safety standards require the fluid to have a stated minimum boiling point for both dry and wet.
It can take some time for the fluid to absorb that much moisture, but variables like humid climate can reduce that, which is why it's commonly recommended to flush once per year to avoid getting close to that wet boiling point. Racers and trackday riders who flush more frequently to combat heat breakdown, are typically doing so when the fluid is much closer to the dry boiling point than the wet.
If you're racing, riding at the track, or building a supermoto (where the front brakes are being asked to do a LOT more than what they were intended) it's highly recommended to buy fluid with a 600+ degree dry boiling point. For more aggressive/expert racing, go with an even higher rated fluid to minimize the frequency of flushing, since the additional heat put into the brakes will only break down the fluid faster.
FLUSHING YOUR FLUID
At a minimum, you should flush (completely replace) your fluid at the beginning of each year to combat the previously mentioned absorption of moisture. Doing so means your fluid will likely not get near that lower wet boiling point.
As water starts to be absorbed, the effectiveness of the brakes will diminish resulting in a spongey lever feel. This can happen sooner in humid climates, so you may need to flush more than once per year to combat this issue.
The other reason you may find a spongey-feeling lever is from the fluid braking down after hard braking (like track riding, stunting, or even hard canyon riding). The more aggressive you are with your brakes, the quicker you'll break down the fluid, thus requiring more frequent fluid changes. If you find yourself at this point, consider jumping to a higher temperature fluid to help extend fluid performance.
You'll also want to ensure your brake components are up to the task for what you are doing. If you're pushing the limits on your sport bike at the track, you may want to upgrade pads, lines, and maybe even a master cylinder. If you're building a supermoto, there is a much higher need to upgrade the front brake components that are meant for lighter use in the dirt!
Please dispose of fluid responsibly!
DOT RATINGS
Most motorcycle brake systems are designed to use DOT 4 or DOT 5.1 fluid. HOWEVER, pay close attention to your brake (and clutch!) components so that you use the correct fluid type. Some aftermarket brake components, like Beringer, come with special seals that are only compatible with DOT 4 fluid. Using incompatible fluid in your hydraulic brake and clutch systems will cause damage to those special seals, and ultimately failure.
SUPERMOTO
Once you've addressed getting proper fluid for your supermoto bike, make sure you have proper components for how you plan to ride your supermoto! Dirt bikes aren't designed for heavy front brake use, so when converting to supermoto, you'll want to upgrade some (or all) of these components to suit how you plan to ride supermoto. A 320mm oversized front rotor and caliper adapter is a great starting point for street riding and even some novice track riding. If you're an aggressive rider looking to race (an even aggressive canyon riding), you'll want to consider upgrading all of the front brake components. This includes a larger master cylinder, 4-piston or larger caliper, and a larger rotor that is also thicker to cope with the additional heat that comes from using a larger brake system.
PUTTING THE CAP ON (This Article)
Hopefully this adds to your knowledge base and adds to the success of your riding and/or racing! Remember to use a high quality fluid that will handle more abuse than you think you'll throw at it, and take care of your brake system with proper flushing of the fluid.
Happy riding!